1. Field of the Invention
The invention relates to the separation of desired components of a gas stream from impurities contained therein. More particularly, it relates to the enhancing of the recovery of high purity product in such gas separation operations.
2. Description of the Prior Art
Permeable membrane systems provide a commercially feasible means for separating and purifying a desired component or components from a feed gas mixture threreof with other less desired components or impurities. For such purposes, the feed gas mixture is passed to a gas permeable membrane capable of selectively permeating certain components of the mixture that are withdrawn from the discharge side of the membrane as a permeate gas at a reduced pressure. The non-permeate portion of the gas mixture is withdrawn from the feed side of the membrane essentially at the feed gas pressure. Typically, the component desired to be purified will be recovered as the permeate portion of the feed gas mixture, while the impurities associated therewith are withdrawn as the non-permeate portion of said feed gas mixture. In applications involving the purification of purge gases from the reaction loop of ammonia or methanol plants, such purge gases are commonly passed to a permeable membrane system at substantially the reaction pressure level. The components desired to be recovered, i.e., hydrogen and nitrogen in ammonia plants, and hydrogen and carbon dioxide in methanol plants, are recovered as permeate gas at a reduced pressure and, after recompression, are recycled to the ammonia or methanol loop for further production of the desired product. The purge gas from the reaction loop also typically contains ammonia or methanol together with various process impurities. Before such purge gas is brought into contact with the permeable membrane, such ammonia or methanol is desirably removed to the extent that such components might be harmful to the permeable membrane so as to affect its effectiveness over its intended operation life. For this purpose, a water wash or distillation process is typically employed, with said ammonia or methanol being removed in aqueous form.
The use of permeable separation membranes in the treatment of ammonia synthesis purge streams to recover hydrogen is taught by Gardner et al, in "Hollow Fiber Permeator for Separating Gases", Chemical Engineering Progress, October, 1977, pages 76 to 78. This approach was affirmed by Perry, U.S. Pat. No. 4,172,885, with reference to the known reaction of hydrogen and nitrogen at a superatmospheric pressure to produce ammonia. As stated therein, an ammonia-containing reaction effluent is withdrawn from the ammonia synthesis reaction zone. This reaction effluent, which contains substantial amounts of unreacted hydrogen and nitrogen, is recycled in a well-known ammonia synthesis loop to the ammonia synthesis reaction zone. Combined with this recycling reaction effluent in the ammonia synthesis loop is a synthesis feed gas stream of hydrogen, nitrogen and impurities such as methane and argon. In addition to the removal of product effluent from the reaction effluent in the ammonia synthesis loop, a purge stream is removed from the reaction effluent in an amount such as to maintain the proportion of impurities desirably low. In the process as described by Perry, the purge stream from the ammonia synthesis is chilled to condense and coalesce ammonia and is then scrubbed with water to provide a purge stream having less than about 0.5 volume % ammonia and water vapor therein. Alternatively, the ammonia content of the purge stream is reduced to less than 0.1 volume % by either chilling to coalesce ammonia, or adsorption or absorption of ammonia. The thus-treated purge stream, substantially at the synthesis reaction pressure level, is thereafter passed to the permeable membrane for permeation of hydrogen therethrough and reintroduction to the ammonia synthesis reaction zone.
It will be appreciated that ammonia removal pretreatment as indicated above necessarily results in the application of coalescers, scrubbers and the like, as well as the need for regenerating ammonia that is removed in the form of aqueous ammonia in order to avoid undue loss of product ammonia. While such an approach can be utilized in commercial operations, the development of improved techniques and systems would be desirable in the art. Thus, it is desirable that, in avoiding contact of the permeable membrane with feed gas components harmful thereto at particular concentrations, the pretreatment process be simplified and the need for employing water scrubbing and ammonia regeneration be avoided. At the same time, the enhanced recovery of the ammonia content of the purge gas is desirable so that the overall product recovery of the ammonia production operation can be enhanced. Those skilled in the art will appreciate that similar concerns and desires pertain to the field of methanol production. Similarly, a desire for such improvements exists with respect to other gas purification operations, not necessarily associated with a reaction synthesis loop, but wherein components of a feed stream harmful to a permeable membrane but otherwise valuable are desirably removed therefrom prior to contact with said membrane. In such operations, it is also generally desired to avoid the loss of such valuable components so that an enhanced recovery of desired, high purity product can be achieved.
It is an object of the invention, therefore, to provide an improved process for the separation of desirable components of a gas stream from impurities associated therewith.
It is another object of the invention to provide a process for the enhanced separation of impurities from a gas stream upon contact with a gas permeable membrane.
It is a further object of the invention to provide a process for facilitating the recovery of desirable components removed from a gas stream prior to the separation of impurities from said gas stream by contact with a gas permeable membrane.
With these and other objects in mind, the invention is hereinafer described in detail, the novel features thereof being particularly pointed out in the appended claims.